blk-mq: don't use the requeue list to queue flush commands

blk-mq: don't use the requeue list to queue flush commands

Currently both requeues of commands that were already sent to the
driver and flush commands submitted from the flush state machine
share the same requeue_list struct request_queue, despite requeues
doing head insertations and flushes not.  Switch to using two
separate lists instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>